High-silicon ferritic compacted graphite cast iron having high-temperature strength and high oxidation-resistance

ABSTRACT

The present invention relates to a high-silicon ferritic CG cast iron having high-temperature strength and high oxidation-resistance. More particularly, the present invention relates to a high-silicon ferritic CG cast iron having high-temperature strength and high oxidation-resistance, which has a heat and crack resistant property under a high-temperature use condition and can be used very suitably for a turbine housing, i.e., a part of a vehicle engine that is subjected to a repeated heating and cooling process. To this end, the present invention provides a high-silicon ferritic CG cast iron having high-temperature strength and high oxidation-resistance, which comprises ion (F) as a main ingredient, 3.00 to 3.60% by weight of carbon (C), 4.00 to 4.80% by weight of silicon (Si), 0.10 to 0.30% by weight of manganese (Mn), 0.07% by weight or less of phosphor (P), 0.02% by weight or less of sulfur (S), 0.30 to 1.20% by weight of molybdenum (Mo), 0.01 to 0.10% by weight of chromium (Cr), 0.2% by weight or less of titanium (Ti), 0.3 to 1.20% by weight of nickel (Ni), 0.30 to 1.20% by weight of vanadium (V) and 0.050% by weight or less of magnesium (Mg).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2007-0114210 filed on Nov. 9, 2007, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a high-silicon ferritic compacted graphite (CG) cast iron having high-temperature strength and high oxidation-resistance. More particularly, the present invention relates to a high-silicon ferritic CG cast iron having high-temperature strength and high oxidation-resistance, which has a heat and crack resistant property under a high-temperature use condition and can be used for a turbine housing, i.e., a part of a vehicle engine that is subjected to a repeated heating and cooling process.

(b) Background Art

Conventionally, a CG cast iron has been manufactured by precisely controlling magnesium inoculation at a nodular cast iron base to facilitate to secure its mechanical and physical properties (e.g. tensile strength). To this end, a precise control device, and high-grade materials phosphor and sulfur contents of which are low have been used. However, such a cast iron producing method is problematic in that there is a possibility of defective material and cast due to their susceptibility to a condition change.

Particularly, one example of the conventional material used to produce the CGT cast iron is a high nickel austenite nodular cast iron containing approximately 4.5 to 5.5% by weight of silicon and 30 to 40% by weight of nickel. This high nickel austenite nodular cast iron, however, exhibits a poor castability, resulting in a high manufacturing cost, and shows a poor machinability, causing a great burden in terms of expenditure.

Another exemplary material used to produce the CG cast iron is a heat-resistant nodular cast iron. But it is insufficient in high temperature strength, and thus is partially used merely in a low exhaust vehicle class.

The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that is already known to a person skilled in that art.

SUMMARY OF THE DISCLOSURE

The present inventors have researched and studied in an effort to solve the above problems occurring in the prior art, and has produced, as a result of the research, a ferritic CG cast iron in which the graphite includes compacted form and nodular or spheroidal form in a certain ratio, and which comprises a chemical composition of C, Si, Mn, Mo, V, Ni, Cr, Ti and Mg the content of each of which is in a particular range with respect to an element Fe. Resultantly, the present inventors have found that the produced ferritic CG cast iron material has an excellent heat resistance in a high temperature atmosphere and meets high-temperature physical property requirements to thereby complete the present invention.

In one aspect, the present invention provides a high-silicon ferritic CG cast iron having high-temperature strength and high oxidation-resistance, in which the graphite therein includes a compacted form of graphite in an amount of 60% or more and a nodular or spheroidal form of graphite as the remaining component, and in which silicon (Si) is contained in an amount of more than 4.0% and a crystallized carbide of vanadium (V) is dissolved in a given amount so as to allow these elements to exhibit their inherent chemical characteristics without segregation of the above elements so that high-temperature oxidation resistance and high temperature strength are increased.

In another aspect, the present invention provides a high-silicon ferritic CG cast iron having high-temperature strength and high oxidation-resistance, which comprises ion (F) as a main ingredient, 3.00 to 3.60% by weight of carbon (C), 4.00 to 4.80% by weight of silicon (Si), 0.10 to 0.30% by weight of manganese (Mn), 0.07% by weight or less of phosphor (P), 0.02% by weight or less of sulfur (S), 0.30 to 1.20% by weight of molybdenum (Mo), 0.01 to 0.10% by weight of chromium (Cr), 0.2% by weight or less of titanium (Ti), 0.3 to 1.20% by weight of nickel (Ni), 0.30 to 1.20% by weight of vanadium (V) and 0.050% by weight or less of magnesium (Mg).

It is understood that the term “vehicle” or “vehicular” or other similar terms as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like. The present cast iron will be particularly useful for a turbine housing as a part of a wide variety of motor vehicles.

Other aspects of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a test result of high-temperature tensile strength for specimens according to an example and a comparative example of the present invention; and

FIG. 2 is a graph showing a test result of high-temperature oxidation resistance for specimens according to an example and a comparative example of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiment of the present invention, examples of which are illustrated in the drawings attached hereinafter, wherein like reference numerals refer to like elements throughout. The embodiments are described below so as to explain the present invention by referring to the figures.

A high-silicon ferritic CG cast iron material according to the present invention is excellent in oxidation-resistance under high temperature conditions as well as in high-temperature mechanical and physical properties, and thus can be used particularly for a turbine housing of a turbo charger.

The high-silicon ferritic CG cast iron according to the present invention exhibits a ferrite substrate structure in a compacted graphite, and includes Si, Mo, V, Ni and Cr added thereto to improve a high-temperature physical property. The high-silicon ferritic CG cast iron comprises iron (Fe) as a main ingredient, 3.00 to 3.60% by weight of carbon (C), 4.00 to 4.80% by weight of silicon (Si), 0.10 to 0.30% by weight of manganese (Mn), 0.07% by weight or less of phosphor (P), 0.02% by weight or less of sulfur(S), 0.30 to 1.20% by weight of molybdenum (Mo), 0.01 to 0.10% by weight of chromium (Cr), 0.2% by weight or less of titanium (Ti), 0.3 to 1.20% by weight of nickel (Ni), 0.30 to 1.20% by weight of vanadium (V) and 0.050% by weight or less of magnesium (Mg).

The reason for limiting components and composition ratio of thereof the CG cast iron according to the present invention will be described hereinafter.

(1) Carbon (C): 3.00 to 3.60% by Weight

The amount of carbon (C) is limited to 3.00˜3.60% by weight in consideration of degradation of fluidity and crystallization of primary graphite.

(2) Silicon (Si): 4.00 to 4.80% by Weight

Silicon (Si) is an element that contributes to crystallization of graphite. The amount of silicon (Si) is limited to 4.00 to 4.80% by weight in consideration of a ferrite process of the substrate, improvement of oxidation resistance, transformation increase effect of austenite in the ferrite, the flow of the melt during the casting process and machinability.

The reason for the above limitation of silicon (Si) is that the high-temperature oxidation resistance is decreased in the range of the amount of 4.00% by weight or less and the development material is weakened to induce brittleness in the range of the amount of 4.80% by weight or more.

More specifically, it is preferably to produce a great amount of Fe₂SiO₄ of on an interface between an oxide film and a substrate so as to increase oxidation resistance, and hence silicon is contained in an amount of 4.00% by weight or more, based on the total weight of the CG cast iron material. Generally, silicon (Si) is an element that increases of oxidation resistance as mentioned above and rises the transformation temperature of austenite in the ferrite. However, if the amount of silicon (Si) exceeds 4.80% by weight, the hardness of the ferrite is increased to reduce ductility. For this reason, the CG cast iron material of the present invention limits the content of silicon (Si) for the purpose of increase of oxidation resistance and prevention of reduction in ductility.

(3) Manganese (Mn): 0.10 to 0.30% by Weight

Manganese (Mn) is segregated in a boundary of eutectic cells and the transformation temperature of austenite is decreased in a ferrite for the boundary to degrade a heat crack resistance. Thus, it is preferably to limit the amount of manganese (Mn) to 0.10 to 0.30% by weight.

(4) Phosphor (P): 0.07% by Weight or Less

Phosphor is a kind of impurity which forms steadite. Thus, it is preferably to limit the amount of phosphor (P) to 0.07% by weight or less.

(5) Sulfur (S): 0.02% by Weight or Less

Sulfur is a kind of impurity that is harmful to nodularization of graphite. Thus, it is preferably to limit the amount of sulfur (S) to 0.02% by weight or less.

(6) Molybdenum (Mo): 0.30 to 1.20% by Weight

Molybdenum is an element that is combined with carbon to bring about formation of precipitated carbide and reduces an average thermal expansion coefficient to decrease generation of heat stress in a high temperature region to thereby improve high-temperature strength. When molybdenum is contained in a large amount, the amount of intercellular carbide is increased to degrade machinability and room temperature elongation. Thus, it is preferably to limit the amount of molybdenum (Mo) to 0.30 to 1.20% by weight.

Molybdenum (Mo) can increase the high-temperature strength due to solid solution strengthening effect in the ferrite.

(7) Chromium (Cr): 0.01 to 0.10% by Weight

Chromium (Cr) is an element that forms chromium-based oxide to improve oxidation resistance. Thus, it is preferably to limit the amount of chromium (Cr) to 0.01 to 0.10% by weight. If the amount of chromium (Cr) is beyond the above range, machinability is degraded at the time of addition of chromium (Cr).

(8) Nickel (Ni): 0.30 to 1.20% by Weight

Nickel (Ni) was added to the CG cast iron to improve room temperature elongation. If the amount of nickel (Ni) is beyond the range of 0.30 to 1.20% by weight, the pearlitization of the substrate is greatly enhanced. Thus, it is preferably to set the upper limit of the amount of nickel (Ni) to 1.20% by weight or less.

(9) Vanadium (V): 0.30 to 1.20% by Weight

Vanadium (V) is an element that can attain an improvement effect (precipitation strengthening by VC) of a high-temperature strength. Thus, it is preferably to limit the amount of vanadium (V) to 0.30 to 1.20% by weight. If the amount of vanadium (V) exceeds 1.20% by weight, decarburization reaction is accelerated to degrade the oxidation resistance.

The high-silicon ferritic CG cast iron has a structure in which vanadium-based carbide (VC) is formed in the substrate by addition of vanadium (V). Such a structure can improve high-temperature strength.

(10) Titanium (Ti): 0.2% by Weight or Less

Titanium (Ti) has an effect on formation of graphite, and thus it is preferably to limit the amount of titanium (Ti) to 0.2% by weight or less.

(11) Magnesium (Mg): 0.050% by Weight or Less

Magnesium (Mg) is an element that is added for the purpose of nucleation and growth promotion of the CGI graphite. Thus, it is preferably to limit the amount of magnesium (Mg) to 0.050% by weight or less in view of nodularization and shrinkage defect of graphite.

The high-silicon ferritic CG cast iron material having the above chemical composition can be produced through a typical casting method by an person having ordinary skill in the art, but is not limited by the present invention.

Such a high-silicon ferritic CG cast iron of the present invention is a cast iron alloy material for high temperature which is excellent in heat resistance and oxidation resistance, and hence can be used in a turbine housing of a turbo charger with a high-power engine.

Now, the high-silicon ferritic CG cast iron will be described in more detail with reference to an example and a comparative example of the present invention, and the present invention is not limited to or by the following example.

EXAMPLE AND COMPARATIVE EXAMPLE

As an example of the present invention, as shown in Table 1 below, a specimen of the high-silicon ferritic CG cast iron was fabricated by a typical casting method so as to measure the physical property of the high-silicon ferritic CG cast iron through a chemical composition consisting of iron (Fe) as a main ingredient, 3.13% by weight of carbon (C), 4.30% by weight of silicon (Si), 0.20% by weight of manganese (Mn), 0.05% by weight of phosphor (P), 0.01% by weight of sulfur (S), 1.20% by weight of molybdenum (Mo), 0.08% by weight of chromium (Cr), 0.66% by weight of nickel (Ni), 0.50% by weight of vanadium (V), 0.2% by weight of titanium (Ti) and 0.03% by weight of magnesium (Mg).

As a comparative example, as shown in Table 1 below, a specimen of a heat-resistant nodular cast iron was fabricated by a typical casting method so as to measure the physical property of the nodular cast iron through a chemical composition consisting of iron (Fe) as a main ingredient, 3.31% by weight of carbon (C), 4.16% by weight of silicon (Si), 0.32% by weight of manganese (Mn), 0.048% by weight of phosphor (P), 0.03% by weight of sulfur (S), 0.65% by weight of molybdenum (Mo), 0.02% by weight of chromium (Cr), 0.03% by weight of nickel (Ni) and 0.026% by weight of magnesium (Mg).

TABLE 1 Chemical composition (wt %) C Si Mn P S Cr Ni Cu Mo Ti V Mg Example 3.13 4.30 0.20 0.05  0.01 0.08 0.66 0.02 1.20 0.2 0.50 0.03  Comp. 3.31 4.16 0.32 0.048 0.03 0.02 0.03 — 0.65 — — 0.026 Example

Test Example

Test of the high-temperature tensile strength and high-temperature oxidation resistance was performed on the specimens according to the example and the comparative example as follows.

The high-temperature tensile strength was tested using typical equipment at a material surface temperature of 800° C. as a strict endurance mode. As a test result of the high-temperature tensile strength, it could be seen from a graph of FIG. 1 that the example exhibits a relatively high high-temperature tensile strength as compared to the comparative example.

It could be seen that the high-temperature strength can be enhanced by a solid solution strengthening effect owing to addition of molybdenum (Mo), as well as by a precipitation strengthening effect by VC owing to addition of vanadium (V). Also, it could be seen that the high-temperature strength can be further enhanced by combined addition of these elements.

Test of the high-temperature oxidation resistance was performed such that the specimens according to the example and the comparative example were fabricated into a square shape having a width of 20 mm, a length of 20 mm and a height of 2 mm, were maintained in a heated holding furnace of 700° C. for 300 hours, and then were taken out of the heated holding furnace so as to be cooled in the air. Thereafter, the cooled specimens were subjected to a short blast to remove an oxide scale and then an reduction amount (g/mm²), i.e., a change of mass per unit area after the oxidation resistance test, was obtained and evaluated. The evaluation result of the reduction amount is shown in a graph of FIG. 2.

As a test result of the high-temperature oxidation resistance, it could be seen from the graph of FIG. 2 that the example exhibits a relatively low reduction amount as compared to the comparative example. The reason for this is that it was desirable to produce a great amount of Fe₂SiO₄ on an interface between the oxide film and the substrate.

As described above, the high-silicon ferritic CG cast iron according to of the present invention has an advantageous effect in that it is remarkably excellent in physical properties such as high temperature strength, high-temperature oxidation resistance, etc., as compared to an existing heat resistant nodular cast iron. In addition, the present invention has an economic effect of being capable of manufacturing a turbine housing which is inexpensive and practical, along with such improvement of the physical properties. Further, the alloy material of the present invention can enhance the high-temperature strength and the high-temperature heat resistance when being applied to the turbine housing of the turbo charger to thereby replace an existing nickel cast iron material.

The invention has been described in detain with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents. 

1. A high-silicon ferritic CG cast iron having high-temperature strength and high oxidation-resistance, which comprises iron (F) as a main ingredient, 3.00 to 3.60% by weight of carbon (C), 4.00 to 4.80% by weight of silicon (Si), 0.10 to 0.30% by weight of manganese (Mn), 0.07% by weight or less of phosphor (P), 0.02% by weight or less of sulfur (S), 0.30 to 1.20% by weight of molybdenum (Mo), 0.01 to 0.10% by weight of chromium (Cr), 0.2% by weight or less of titanium (Ti), 0.3 to 1.20% by weight of nickel (Ni), 0.30 to 1.20% by weight of vanadium (V) and 0.050% by weight or less of magnesium (Mg). 